This invention relates generally to package wrapping machine utilizing thin, stretchable film to wrap and display articles contained in trays and, more particularly, to an improved package sensing system for generating signals representative of the size characteristics of packages to be wrapped and a film control system for selecting the width and length of a section of stretchable film to wrap a sensed package in response to such signals.
A variety of film wrapping machines are known in the prior art. One commercially available machine includes a package sensing system which determines the length and the width of a package to be wrapped. Based on the sensed length and width characteristics, the wrapping machine selects one of two differing width film rolls and the length of the film sheet drawn from the selected roll.
Herein, the length of a package refers to the dimension of the package as it is fed into a wrapping machine and the width of the package refers to the dimension of the package perpendicular to the feed of the package into the machine. Thus, when referring to the length and width of a package, the "width" of the package is normally longer than its "length".
In the known commercially available wrapping machine, the length of the package is determined by a first sensor located in the surface of a package feed-in tray and approximately on the center line thereof. The first sensor is utilized to determine the length of a package by fixing the time when the package contacts the sensor relative to the phase of the machine cycle. The width of the package is determined by means of a second sensor similarly located in the surface of the feed-in tray but offset from the center line of the tray by a defined distance. If the second sensor is activated, it is presumed that a wide package has been fed into the machine.
While this wrapping machine is an improvement over earlier prior art machines which utilize a fixed length of a single width film to wrap all packages regardless of size (see for example, U.S. Pat. Nos. 3,662,513 and 3,967,433), the sensing system is limited. To partially overcome the sensing system limitations, a separate mechanical package centering device is provided. This device adds cost to the machine and in the event it is disabled or malfunctions, the centering system may produce erroneous readings. In particular, if an operator inadvertently places a package off center toward the second sensor in the feed-in tray, a narrow package could engage the second, wide package sensor and erroneously indicate that a wide package was to be wrapped with the wider of the two differing width films.
In the event of such an error, the machine should still wrap the package, however, the wrap would be inefficient in terms of film usage and could create bulky, unattractive packages. On the other hand, if a wide package was inadvertently placed toward the side of the feed-in tray away from the wide package sensor, a wide package could be wrapped with narrow film. In this case, the package would probably be inadequately wrapped. Such errors could require rewrapping of packages and could result in contamination or jamming of the machine with resultant down time. Furthermore, the package sensing system of the known wrapping machine only senses the length and width of packages to be wrapped.
It is, thus, apparent that the need exists for an improved package sensing system for measuring the size characteristics of packages to be wrapped and a film control system for selecting the width and length of film to be used to wrap sensed packages. The film width and length to be used for wrapping a given package can then be more accurately selected to improve the wrapping characteristics and efficiency of the film wrapping machine.